Perceptually Lossless Color Compression is desirable in the field of digital imaging, in particular, with application to improved performance of network and mobile imaging devices. Conventional systems provide either lossy or lossless compression, neither of these modes being adequate for most digital imaging or remote capture applications. Lossless compression in colorspace generates very large file sizes, unsuitable for distributed databases and other applications where transmission or hosting size is a factor. Lossy compression assumes an implicit tradeoff between bitrate and distortion so that the higher the compression, the greater the level of distortion. One example conventional compression method is MRC compression. (See Mixed Raster Content (MRC) Model for Compound Image Compression, Ricardo de Queiroz et al., Corporate Research & Technology, Xerox Corp., available at http://image.unb.br/queiroz/papers/ei99mrc.pdf, and see U.S. Pat. No. 7,110,137, the entire disclosures of which are incorporated herein by reference.) In MRC coding, there is typically a foreground layer, a background layer, and a mask (or selector layer) which has a binary value that determines whether a pixel in the image is given the color value of the foreground or background layer. With respect to business documents, the mask layer is an important layer, which generally contains the text and line art information. Since the mask layer is so important, it is generally encoded with the same resolution as the original image, even though this may have a high byte cost. In fact, for very low resolution images, the mask layer may even be of a higher resolution than the original image. The foreground layer assigns a color to those pixels that the mask places in the foreground, while the background layer assigns a color value to the remaining pixels. Both the foreground and background layers are generally encoded at a lower resolution than the original image.
However, what is desired, in fact, neither of these methods achieves: a very high compression rate with no perceptual distortion. Many applications and devices, including digital copiers and MFPs, need a high degree of compression on color scanned documents. For documents to be used effectively as e-mail attachments or in a web-hosted environment, lossless compression is not viable. Even for sensitive medical applications, including CAT and MRI scans, lossless compression is not used in the industry. However, for many applications, including corporate document record archiving, no perceptual loss can be tolerated as these image documents are often treated as the documents of record.